It is known that in metallic materials, for example, austenitic stainless steels that are often used in nuclear power generation plants and chemical plants, stress corrosion cracking likely occurs when tensile stress and corrosive substances exist at the same time. In the case where a pipeline is constructed by connecting a number of tubes (called "pipes" hereinafter) particularly by welding, the surfaces of the pipes at their welded joints are apt to be left in a state of tensile stress because of the thermal influence of welding.
Various techniques are known in the art for improving the stress state at the inner surface of a pipe. One is a method by which a compresive residual stress is given to the inner surface by induction-heating the pipe and simultaneously supplying cooling water through the pipe, thus creating a temperature difference across the wall of the pipe.
To improve the residual stress state in the outer surface of such pipes, however, no practical methods have been available because it is difficult to apply cooling water to the outer surface.
The principles of producing compressive residual stress in the outer surface of a pipe are known: by heating the pipe slowly from its outer surface, and on completion of heating, by cooling the surface, there is produced a compressive residual stress therein.
However, one important technical point here is that unless the cooling is conducted uniformly over the surface concerned, non-uniform distribution of compressive stress will result.